1. Field of the Invention
This invention relates to an oxygen sensor. More particularly, the present invention relates to an oxygen sensor having improved heat resistance and response due to improved electrodes.
2. Description of the Related Art
Oxygen sensors have been used widely to control the fuel cost in various combustion furnaces, to control an air-fuel ratio to allow a ternary catalyst for processing an exhaust gas of an automobile to function effectively, and to control an oxygen concentration in a metal refining process. A semiconductor type (titania type) oxygen sensor which utilizes electron conductivity of oxide semiconductors and a solid electrolyte type (zirconia type) oxygen sensor utilizing ion conductivity of a solid electrolyte are known as such oxygen sensors.
The semiconductor oxygen sensor detects the oxygen concentration in the exhaust gas by utilizing the property of the titania device as an oxide semiconductor in that its electric resistance changes depending on the oxygen concentration, or in other words, its electric resistance is as high as an oxide insulating material when the oxygen concentration is high, but becomes low because oxygen in the oxide disappears when the oxygen partial pressure is low, and the resistance value drops due to the resulting lattice defect of oxygen.
On the other hand, the solid electrolyte oxygen sensor uses zirconia (ZrO.sub.2) stabilized by calcia (CaO), yttria (Y.sub.2 O.sub.3), etc. as an oxygen ion conductive solid electrolyte, and has electrodes on both surfaces of this solid electrolyte, that is, the surface coming into contact with a reference gas and the surface coming into contact with a measured gas. The measured gas such the exhaust gas, for example, is brought into contact with one of these electrodes and the reference gas such as air, for example, is brought into contact with the other electrode. Conduction of the oxygen ions does not occur under the state where the oxygen concentration of the measured gas is substantially equal to the oxygen concentration of the reference gas, and no voltage develops between these electrodes.
However, the oxygen ions move from the reference gas side to the measured gas side inside the solid electrolyte when the oxygen concentration of the measured gas is low. At this time, the oxygen molecule carries the electrons from the electrode on the boundary surface between the platinum electrode and the solid electrolyte on the reference gas side, and the electrons are entrapped as the oxygen ions into the solid electrolyte. As a result, the reference gas side is charged to positive. On the other hand, the oxygen ions moving inside the solid electrolyte emit the electrons at the electrode on the measured gas side, and become oxygen atoms. The oxygen atoms occurring in this way on the measured gas side are consumed as they react with carbon monoxide and hydrocarbons and are emitted in the oxygen molecule state. In this way, the voltage develops between the two electrodes, and the oxygen concentration is detected by utilizing this electromotive force.
As described above, the electrodes for electrically picking up the detection signal corresponding to the oxygen concentration are disposed on the surface of the sensor device in all of the various types of oxygen sensors described above. The electrode has been produced in the past by applying a precious metal paste such as a platinum (Pt) paste to a substrate and baking the substrate (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 3-264857).
In such an oxygen sensor, the electrodes must be made porous in order to allow a flow of the oxygen gas. Further, the boundary surfaces between the electrode, the substrate and the reference gas or the measured gas must be increased. When the electrode is produced by applying the precious paste, however, porosity cannot be increased to a high level, and the problem develops in that sintering of the precious metal particles proceeds depending on the baking temperature and the boundary surface with the measured gas, that is, the operation area, drops. According to a chemical plating method, on the other hand, a porous electrode can be formed, it is true, but the formation of a thick electrode is difficult, and only an electrode having a thickness of about 1 to about 2 .mu.m can be formed. If the electrode has such a limited thickness, sintering occurs because the temperature of the measured gas is as high as 700 to 800.degree. C. and the operation area drops.